1. Field of the Invention
The present invention relates to a method of manufacturing a semiconductor device, and more particularly, to a method of controlling a threshold of a MOS transistor and improving the reliability of a gate insulating film of the MOS transistor.
2. Description of the Related Art
Conventionally, when a MOS transistor is manufactured on a semiconductor substrate, the following steps have been used to control the threshold of the MOS transistor.
First, as shown in FIG. 2A, an element separating film 10 and a gate insulating film 11 are formed on a silicon semiconductor substrate 9 by a well-known technique.
Next, as shown in FIG. 2B, for the purpose of controlling an impurity concentration of a channel region so that the threshold of the MOS transistor is made a desired one, an impurity is injected into a portion under the gate insulating film by a well-known technique, for example, an ion implantation method.
Further, in the case where it is necessary to raise the reliability of the MOS transistor, as shown in FIG. 2C, a treatment for improving the reliability of the gate insulating film is carried out by a well-known technique.
Next, as shown in FIG. 2D, a gate electrode film 12(a) is formed by a well-known technique.
Subsequently, as shown in FIG. 2E, the gate electrode film 12(a) is patterned so that a gate electrode film 12(b) of the transistor is formed by etching removal, and subsequently, a source/drain 13 of the transistor, an interlayer insulating film 14, a contact hole 15, and a metal wiring line 16 are formed by a well-known technique, so that the MOS transistor is manufactured.
In the conventional manufacturing method, the step of controlling the threshold of the MOS transistor and the step of improving the reliability of the gate insulating film are independent from each other, and there has been a problem that the number of manufacturing steps becomes large in order to obtain a highly reliable MOS transistor.
An object of the present invention is to improve a manufacturing method of a semiconductor device to thereby eliminate the above-mentioned problem.
According to a manufacturing method of the present invention, for the purpose of controlling a threshold of a MOS transistor, a nitriding treatment to a gate insulating film is used, with the result that both improving the reliability of the gate insulating film and controlling the threshold of the MOS transistor can be attained by one process step.